The invention relates to a method for making a stent coated with antithrombotic reagents, of filiform material resorbable by the body.
In balloon angioplasty of peripheral, renal and coronary arterial stenoses, a laceration of the vascular innermost skin, i.e., a dissection of the intima, may occur, which ultimately may lead to a vascular occlusion. Available as therapeutic measures are then an emergency bypass operation or the implantation of a vascular prothesis, or vascular support, of a so-called stent in the vessel. Such stents are tubular and reticulate implants inserted in the damaged vessel at reduced diameter and expanded to their final diameter only at the point in the vessel that is to be supported. The stent expands either due to its innate elasticity (so-called self-expanding stents) or expansion is brought about by a balloon which can be inflated inside the stent (so-called balloon-expandable stents). Prior stents consist normally of metal, for instance surgical refined steel, tantalum or nitinol; more recently, also stents of polymers have been proposed.
EP 0 466 105 A2 describes a vascular prosthesis which, however, from its structure, is not intended as a stent but more so as a vascular replacement. This vascular prothesis is not dilatable and, therefore, cannot be advanced such as a stent, by means of a catheter, through a blood vessel to a damaged site and positioned and dilated there. Furthermore, the core of this prior art document is constituted by providing a biocompatible interior surface of a vascular prothesis. Proposed for that purpose is the use of biological material, notably of collagen and elastin fibers, for making the inner coat. To stabilize this inner "biological" coat, a second, synthetic ply is externally applied on said coat.
U.S. Pat. No. 5,061,275 claims a stent fabricated of metal filaments. While plastic and biocompatible materials are mentioned also as stent material, only the fabrication of a stent of metal filaments of a specific alloy is described as the disclosed embodiment. This stent is subjected to a thermal treatment at 520.degree. C., thereby fixing the metal filaments in their new form.
A considerable disadvantage of the prior stents, notably those of metal, is that they either remain as a permanent foreign body in the treated vessel or must be removed again in the course of a further operation.
To eliminate this problem, stents have been proposed which are made of material which the body can resorb. EP 0 428 479 A1 describes a stent of polycaprolactone, e.g.; also known are stents of poly-L-lactic acid, a material resorbable as well by the body (refer to Agrawal, C. M. et al. "Evaluation of Poly(L-lactic acid) as a material for intra-vascular stents"; Biomaterials, 13 (3), 176-182; 1992.
Common to all of the aforementioned stents is a pronounced thrombogeneity which, despite intensive medicamentous anticoagulation, may lead to a subacute stent thrombosis with formation of an infarct (with stents in coronary arteries). Meant by medicamentous anticoagulation is here the administration of anticoagulant substances which propagate throughout the entire system of blood vessels and, thus, result in anticoagulation in the entire system of blood vessels. A disadvantage of medicamentous anticoagulation is that wound healing problems occur also at the insertion site of the stent, mostly in the thigh or groin area, since blood coagulation is being suppressed also at this site.
EP 0 528 039 A1 describes a stent of filiform resorbable material, which stent is made by braiding and may be coated with antithrombotic reagents. Disadvantageous on the fabrication method of this stent is that the filiform braiding is prior to the thermal treatment removed from the forming cylinder, with the result that the stent shrinks during the subsequent thermal treatment.
DE-PS 28 31 360 describes the heparinization of a surface of a medical object wetted by the blood and which, however, does not consist of resorbable material. Arising from the resorbability of the material, though, are specific problems in conjunction with a heparin coating, and at that, notably a swift release of the heparin coating in the course of resorption. This is also why in using resorbable material it is mostly proposed to work the heparin into the resorbable material (refer to EP 0 528 039 A1), instead of applying it externally in one coat.